On any given day, an individual experiences various external stimuli that can provoke a wide range of responses. The responses of the individual can manifest in mental and emotional states, facial expressions, body language, and so on. The stimuli are perceived through one or more senses and can be visual, aural, olfactory, tactile, and so on. Alone or in combination, the stimuli can invoke strong emotions and mental states in the individual who experiences them. Not all individuals experiencing the one or more stimuli will react in a similar manner. That is, when a group of individuals experiences the stimuli, the reactions can be at times similar, at other times widely varied, and so on. How an individual reacts to experienced stimuli can be important to defining the essence of an individual. Furthermore, the individual's responses to the stimuli can have a profound impact on the mental states experienced by the individual.
The mental states that an individual can have in response to external stimuli can vary depending on time frames; such as one time of day versus another, one day to another, and so on. An individual's mental state is important to general well-being and impacts her or his perception of the surrounding environment, decision-making processes, and so on. Multiple individuals' mental states that result from a common event can carry a collective importance, where the collective importance can be, in certain circumstances, even more significant than the importance of each individual's mental state. The mental states of an individual or a group of individuals can vary widely, ranging from happiness to sadness, from contentedness to worry, and from calm to excitement, to name only a few possible states. Despite how critical and influential one's mental states are to daily life, the mental state of a single individual, let alone those of a group, might not always be apparent, even to that individual. The ability and means by which one person perceives her or his emotional state can be quite difficult to summarize. Though an individual can often perceive her or his own emotional state quickly, instinctively, and with little or no conscious effort, the individual might encounter difficulty when attempting to summarize or communicate her or his mental state to others. This difficulty of understanding and communicating mental states becomes far more complex when the mental states of multiple individuals are considered.
Gaining an insight into the mental states of one or more individuals is an important technique for understanding how people react to various external stimuli such as views of the natural landscape, political and sports events, educational programs, natural disasters, etc. However, proper interpretation of mental states is very difficult when the individuals being considered are themselves unable to accurately identify and communicate their mental states. The identification and communication of mental states can be further complicated by the fact that multiple individuals can have similar or very different mental states when taking part in a communal activity. For example, the mental state of two friends viewing an important political debate can be disparate—if one friend is a supporter of the winning candidate, while the other friend is a supporter of the losing candidate, it is reasonable to expect widely varying mental states between the two friends. However, the problem of defining the resulting mental states from multiple people experiencing complex stimuli can be a considerably complicated exercise.